1. Field of the Invention
The present invention relates to an improvement in an optical disc and an injection compression molding die for producing an optical disc substrate for the optical disc, and in particular to an optical disc that does not cause damages on a printing plate during screen printing on a printable plane of such disc, and an injection compression molding die for producing a substrate for such disc.
2. Description of the Related Art
Molding of a thin disc for lamination, such as that being used for DVD (Digital Versatile Disc), is generally carried out by the injection compression molding method in order to satisfy standards on birefringence or skew. A conventional die used for such injection compression molding will be explained referring to FIGS. 8 to 9.
FIG. 8 is a partial sectional view showing a tightened state of a conventional injection compression molding die for producing an optical disc substrate, FIG. 9 is a partial sectional view showing a tightened state of another conventional die used for such injection compression molding for producing an optical disc substrate, and FIG. 10 is a partial sectional view showing a structure of an optical disc using a substrate produced by the injection compression molding die shown in FIGS. 8 and 9.
First, a constitution and structure of the injection compression molding die 10 shown in FIG. 8 will be described. The injection compression molding die 10 mainly comprises a fixed-side die 1010 composed of a mounting plate 1011 and a signal-side mirror 1012 mounted thereon, and a mobile-side die 1020 composed of a mounting plate 1021 and a readout-side mirror 1022 mounted thereon. The mounting plate 1011, the signal-side mirror 1012, the readout-side mirror 1022 and the mounting plate 1021 are arranged in a concentric manner. On an outer periphery of the signal-side mirror 1012, a fixed-side interlock ring 1030 is located so as to be mounted on the mounting plate 1011. To such fixed-side interlock ring 1030, a cavity ring 1040 is joined and attached so as to allow free sliding on the outer peripheral plane of the readout-side mirror 1022, while being constantly pressed toward such readout-side mirror 1022 by means of a spring not shown in the figure. A sprue bush 1050 is located at the center hole of the fixed-side die 1010, and opposing to the sprue bush 1050, a punch 1060 is located at the center hole of the mobile-side die 1020. Now reference numerals 1013, 1051, 1023 and 1061 denote temperature controlling circuits for the signal-side mirror 1012, the sprue bush 1050, the readout-side mirror 1022 and the punch 1060, respectively; a reference numeral 1024 denotes a degassing hole; and a reference numeral 1062 denotes a thrusting member.
The conventional injection compression molding die 20 shown in FIG. 9 mainly comprises a fixed-side die 2010 composed of a mounting plate 2011 and a signal-side mirror 2012 mounted thereon, and a mobile-side die 2020 composed of a mounting plate 2021 and a readout-side mirror 2022 mounted thereon, in the similar manner as the injection compression molding die 10. On an outer periphery of the signal-side mirror 2012, a cavity ring 2040 engaging with a conical plane formed on the outer periphery of the readout-side mirror 2022 is joined and attached while being constantly pressed toward such readout-side mirror 2022 by means of a spring not shown in the figure. A reference numeral 2030 denotes a fixed-side interlock ring. A sprue bush 2050 is located at the center hole of the fixed-side die 2010, and opposing to the sprue bush 2050, a punch 2060 is located at the center hole of the mobile-side die 2020. Now reference numerals 2013, 2051, 2023 and 2061 denote temperature controlling circuits for the signal-side mirror 2012, the sprue bush 2050, the readout-side mirror 2022 and the punch 2060, respectively; and a reference numeral 2062 denotes a thrusting member.
When an optical disc substrate is molded using thus composed injection compression molding die 10 or 20, a stamper S is mounted on the signal-side mirror 1012 or 2012, a molten resin is injected from the sprue bushing 1050 or 2050 into a cavity C surrounded by the stamper S. the readout-side mirror 1022 or 2022 and the cavity ring 1040 or 2040, to effect compression molding. This produces an optical disc substrate which is used for an optical disc D, an enlarged view of which is partially shown in FIG. 10.
The optical disc substrate thus produced using the conventional injection compression molding die 10 or 20, however, has on the outer periphery thereof a burr B with a height less than 10 xcexcm. This is because the readout-side mirror 1022 and the cavity ring 1040 are kept in slide contact in the injection compression molding die 10, or the readout-side mirror 2022 and the cavity ring 2040 are kept in engagement in the injection compression molding die 20. Depending on the structure of the die, the burr B may have a sharp-edged tip as shown in FIG. 10. The optical disc D having such burr B may undesirably make scratches on a printing plate P during screen printing on the printable plane of such optical disc D, which significantly reduces the service life of the printing plate P, and requires frequent exchange thereof. Alternatively, removal of the burr B before printing is required, which will however degrade the yield and thus push up the product cost of the optical disc.
An approach to reduce generation of the burr B requires an extremely high level of accuracy of the die structure, which will undesirably push up the production and maintenance costs of the die and significantly worsen the operation ratio of a molding machine.
To solve the foregoing problem, it is therefore an object of the present invention to provide an optical disc capable of preventing scratches from being produced on the printing plate with no special need of raising accuracy of the die, and to provide an injection compression molding die for producing such a disc substrate.
Hence a first aspect of the invention solves the foregoing problem by forming a ring-formed projected portion on an outer periphery of a printable plane of an optical disc.
In a second aspect of the present invention, the projected portion of the optical disc as described above has a continuous form.
In a third aspect of the present invention, the projected portion of the optical disc as described in the first aspect has a height of 10 xcexcm to 50 xcexcm.
In a fourth aspect of the present invention, the projected portion of the optical disc of the first or the second aspect of the present invention is formed in the inside and/or outside vicinity of a ring-formed burr generated on the outer periphery of the disc.
In a fifth aspect of the present invention, a ring-formed recessed portion formed on the outer periphery of the printable plane of the optical disc is formed so as to house a ring-formed burr.
Moreover, in a sixth aspect of the present invention, the foregoing problem is solved by an injection compression molding die which comprises a signal-side mirror; a readout-side mirror; a cavity ring joined to the signal-side mirror, and sliding on or being engaged with an outer periphery of the readout-side mirror; a sprue bush located at the center of the signal-side mirror; and a punch located at the center of the readout-side mirror; in which a molten resin is injected from the sprue bush into a cavity surrounded by a stamper mounted on the surface of the signal-side mirror, the surface of the readout-side mirror and an inner peripheral plane of the cavity ring, and compressed to be transferred with pits from the stamper, and is punched out by the punch to form a center hole of an optical disc substrate; wherein the surface of the readout-mirror and/or the inner peripheral plane of the cavity ring has a ring-formed recessed groove in an area around where the readout-mirror and the cavity ring come into proximity.
According to the optical disc of the first aspect of the invention, the printing plate P can be raised upward by means of the ring-formed projected portion.
According to the optical disc of the second aspect of the invention, warping of the disc can be prevented, which is an operation additional to those expected from the optical disc of the first aspect.
According to the optical disc of the fourth aspect of the invention, operations equivalent to those of the first and the second aspects will be obtained.
According to the optical disc of the fifth aspect of the invention, the burr is prevented from being brought into contact with the printing plate P owing to the ring-formed projected portion.
Moreover, according to the injection compression molding die of the sixth aspect of the invention, the printing plate P will successfully be prevented from being damaged without upgrading the accuracy of the die constitution.
As has been described in the above, the present invention can provide an optical disc substrate not causative of damages on the printing plate. This will successfully result in:
1. longer service life of the printing plate, and reduction in the number of production of the printing plate;
2. reduction in time for exchanging the printing plate, and reduction in the number of process steps for checking thereof before and after the exchange;
3. reduction in the molding downtime for exchanging the printing plate; and
4. reduction in disposal shot before and after the exchange of the printing plate.